1. Field of the Invention
This invention is related to the general field of sublimation and heat transfers and, in particular, to an improved machine for transferring images onto cylindrical mugs.
2. Description of the Prior Art
Sublimation is the physical transformation of a solid directly into vapor at a certain pressure and temperature, bypassing the liquid state in the process; it occurs in the case of some substances, such as dry ice, that have a lower vapor pressure in liquid than in solid form. Since various kinds of inks with this property have been discovered, the process of sublimation has been used to imprint images on articles of manufacture directly from sublimable, image-bearing, dyes printed on a substrate. The image on the substrate is placed in direct contact with a surface capable of receiving and retaining the vaporized dyes and it is heated to a temperature at least as high as the sublimation temperature of the various inks constituting the image. Thus, the image is transferred directly onto the surface of the article by vaporization of the dyes constituting the image and by their immediate absorption into the polymeric coating.
A similar process occurs by thermal transfer of dyes capable of diffusing from an image bearing substrate into the polymeric fabric of the coating in the receiving surface. The same equipment and general operating conditions and issues apply to both processes.
Since all substances in vapor form tend to expand, a true-image transfer can be achieved only if the sublimated dyes are captured by the polymeric coating as soon as they vaporize. Therefore, direct contact under pressure between the dyes and the coating is essential for a good-quality transfer. This in turn requires a high operating temperature because the temperature of sublimation of a substance is directly related to the pressure to which the substance is subjected. Thus, in order to improve the sublimation transfer process, a high transfer pressure and, correspondingly, a high operating temperature must be utilized, among other factors. In addition, both pressure and temperature must be uniformly distributed over the transfer surface in order to avoid blurs and distortions in the transferred image that may result from hot or cold spots and from non-uniform contact between the image-bearing substrate and the receiving surface. Typically, sublimation transfers to ceramic and glass surfaces have been performed by applying heat to the bearing substrate for 10 to 20 seconds at temperatures between 200 and 450 degrees Fahrenheit and pressures from 2 to 30 psig.
These general principles have been utilized to develop several processes and equipment for transferring images to the surface of mugs and similar articles by sublimation. For example, U. S. Pat. No. 4,842,613 to Purser (1989), U. S. Pat. No. 4,844,770 to Shiraishi et al. (1989) and U. S. Pat. No. 4,923,848 to Akada et al. (1990) describe various techniques and equipment to effect sublimation transfers. In particular, U. S. Pat. No. 4,874,454 to Talalay et al. (1989) discloses a sublimation transfer device that utilizes a flexible heating pad that is pressed against the curved surface of a mug by an enveloping tensioning belt. Similarly, U. S. Pat. No. 4,943,684 to Kramer (1990) describes a sublimation machine comprising two heated jaws for clamping and transferring images to the two sides of a cylindrical mug.
The main problem encountered with existing equipment is the quality of the image resulting from the sublimation transfer. Fading of colors, distortions, lack of sharpness, all result from uneven pressure and temperature distributions over the transfer surface, which in turn depend on the physical structure and functional features of the apparatus used to effect the transfer. For example, the device described in the Talalay et al. patent operates on the principle of a transfer substrate pressed against the receiving surface of a mug by an enveloping belt under tension, as illustrated in FIG. 1. The mug M, seen in top view, is inserted into the loose cuff C formed by the belt B when no tension is applied to it. A heater H and an image-bearing substrate S line the inside surface of the cuff, so as to be in direct contact with the outside surface of the mug, onto which the image is to be transferred. The belt B is looped around two supporting rollers R attached to the frame of the device, and the belt can be tightened by applying a tensioning force on one end of the belt (as in the direction of the arrow A1), or loosened by releasing the force, while the other end of the belt is anchored to a fixed support F. In operation, the belt B is tightened to induce pressure on the image in contact with the mug's surface and the heater is actuated to cause the transfer by sublimation.
From the structure just described, it is easy to see that the pressure on the transfer surface results from the shortening of the belt B over the fixed circumference of the mug M. Given the fixed position of the rollers R and of the support F, the pressure at each point on the surface of the mug enveloped by the belt is produced by the single-source tensioning force pulling the belt, the active component of which is a force in the direction of arrow R. Thus, the actual pressure exercised by the interior surface of the belt at any point on the surface of the mug is measured by the perpendicular force resulting at that point from the effects of the friction between the two surfaces as the belt is tensioned, from the deformation of the belt as it stretches under tension, and from the ability of the layers lining the belt to conform to the surface of the mug. Obviously, a very complicated set of forces is at work while the mug is being compressed by the belt through the application of a single-source tensioning force, and the best one can hope for is a uniform approximation to a constant pressure distribution. In addition, the stresses imparted to the heater H and the substrate S by the repeated tensioning and releasing of the belt cause them to develop creases that effect the heat distribution over the image, as well as the pressure distribution, thus affecting the quality of the transfer. Therefore, there still exists a need for a sublimation transfer device that achieves a more uniform distribution of pressure and temperature over the transfer surface to obtain a sharper and unblemished image.